Implement a extension API

[arunoda]

We only had a way to add decorators to a local story kind.
With this PR, we are bring global decorators and a new addon API.

With that addon API, we'll be able to add new APIs to create stories. Just like below:

stories.addWithInfo(
  'Basic Modal',
  `
    The \`<Story>\` component can be used to show additional information with
    your story.
  `,
  () => (
    <Button>The Button Text</Button>
  )
);